1. Field of the Invention
The present invention, generally, relates to a method for forming metal wirings that can be used favorably in the creation of fine semiconductor elements. More specifically, the present invention relates to a method for forming a metal film by atomic layer deposition using a β-diketone metal complex.
2. Description of the Related Art
Next-generation fine semiconductor elements require formation of semiconductor elements having not only two-dimensional structures, but also three-dimensional structures. Accordingly, the formation of metal wirings for such fine semiconductor elements also requires the ratio of wiring height to wiring width to be improved to a range of 5 to 10, and the ratio of height to hole diameter to a range of approx. 5 to 10 for connection holes used to connect top-layer wirings and bottom-layer wirings. For this reason, it becomes necessary, when forming copper wirings for use as these metal wirings, to form a thin metal film that not only provides a copper diffusion barrier to prevent diffusion of copper, but also achieves good shielding property.
To this end, wiring structures having such high aspect ratios are creating a need to form metal films using the CVD method and ALD method known to achieve good shielding property, instead of forming metal films using the conventional PVD method. In particular, metal films that serve as Cu diffusion barriers must be thin, low in resistivity in order to prevent an increase of the wiring resistance, and formed in a manner ensuring good shielding property in areas of high aspect ratios, and these requirements necessitate use of the CVD method and ALD method to form these metal films. Among others, metal films that serve as Cu diffusion barriers for Cu wirings have traditionally used Ta/TaN films produced by PVD. With the PVD method, however, it is difficult to form barrier films having a uniform thickness over wirings of high aspect ratios or via holes and contact holes used for connecting wirings, and instead the PVD method tends to produce discontinuous films on side faces of trenches, via holes and contact holes, especially on side faces at the bottom of height gaps, as the film thickness decreases. As a result, Cu films formed by the PVD method on top of these barrier films also tend to become discontinuous. For this reason, voids are generated in electroplated copper layers where these discontinuous Cu films are used as seeds, which makes it difficult to form good wirings.
As a countermeasure, a method is currently reported in the area of Cu film forming using the PVD method, where a CVD-Co film is formed on top of a PVD-TaN film, instead of forming a PVD-Ta/TaN film under the traditional method, after which a Cu film is formed to form a continuous Cu film (U.S. Pat. No. 6,365,502). Since a smoother PVD-Cu film can be easily formed on a CVD-Ru film compared to on a PVD-Ta/TaN film, good burying property can be achieved at the time of Cu plating. Even when a film produced by the PVD-Cu process is used as a seed layer, therefore, good Cu plating can be achieved by forming a Co film or Ru film as the foundation. Among the film deposition methods using organic metal materials, the chemical vapor deposition (CVD) method produces films having higher resistivity than bulk films because of the effects of hydrocarbons, etc., breaking away from organic ligands and remaining in the film. In the case of the CVD method, high impurities and consequent low surface smoothness make it generally difficult to form a continuous film when the film thickness is reduced. Furthermore, it is difficult to achieve good adhesion when a Cu film is formed on top of the aforementioned film lacking in continuity. Although the CVD method normally achieves good shielding property under general conditions, when finer elements are needed and thus the aspect ratios of wiring trenches and connection holes must be increased substantially, it is difficult to shield the wirings effectively. Accordingly, the atomic layer deposition (ALD) method is desirable in situations where the film thickness must be reduced but the required shielding property must also be achieved. Under the ALD method, material molecules are adsorbed onto the surface in a uniform manner, after which the adsorbed molecules are caused to react with a different gas to break down and thereby eliminate the ligands, in order to form a desired metal film. With this method, however, finding a proper combination of a type of molecule that can be adsorbed to achieve good shielding property on one hand, and a type of reactant gas capable of easily breaking down the ligands of such molecule on the other, is difficult, and normally the resulting film tends to contain impurities such as residual carbon, oxygen and nitrogen and its resistivity is also likely much higher than the intended level. Under U.S. Pat. No. 6,527,855, a method to form a Co film using an organic metal material of Co and hydrogen gas is described, wherein oxygen gas is supplied after hydrogen gas is supplied. Under this method, however, the formed Co film tends to contain residual carbon and oxygen from the organic metal material and it is therefore difficult to obtain a quality Co film offering low resistivity.
Also, a Cu film required in electroplating as a seed layer must be formed thinly and continuously, just like a Co film. Particularly when a finer element is needed and thus an attempt is made to form a Cu film by the conventional PVD method inside a via hole of 20 to 30 nm in size, only a discontinuous Cu film is formed inside the via hole. If conditions are changed to form a continuous Cu film, on the other hand, the opening of the via hole is buried by the Cu film and voids generate inside the hole. These situations also present the need to form a thin, continuous Cu film.